Boundary Independent Broadcasts in Graphs

A broadcast on a nontrivial connected graph G is a function f from the vertices of G to the non-negative integers such that f(v) does not exceed e(v) (the eccentricity of v) for each vertex v. If G is disconnected, we define a broadcast on G as the union of broadcasts on its components. In a search for the best way to generalise the concept of independent sets in graphs to independent broadcasts, there are several ways to look at an independent set X of a graph G. One way is from the point of view of the vertices in X: no two vertices are adjacent -- the usual definition. Another way is from the point of view of the edges of G: no edge is incident with (or covered by) more than one vertex in X. Using the latter approach we define boundary independent broadcasts as an alternative to independent broadcasts as defined by D. Erwin [Cost domination in graphs, Doctoral dissertation, Western Michigan University, 2001], which we refer to here as hearing or h-independent broadcasts. We compare the boundary independence broadcast number to the independence number and the h-independence broadcast number and show that the differences can be arbitrary, while the ratios are bounded; the bounds we present are asymptotically best possible. We also show that, although the difference between the boundary independence broadcast number and the independence number can be arbitrary for trees, they are equal for any 2-connected bipartite graph. We prove a tight upper bound for the boundary independence number and characterise graphs for which equality holds. Using this bound and the established ratio we obtain a new tight upper bound for Erwin's h-independence broadcast number.